In areas other than motor vehicle technology, analysis-measuring apparatus are known with which the vapor pressure of fuel can be determined under laboratory conditions. Methods of this kind are the vapor pressure determination according to Reid as set forth in DIN 51754/ASTM D 323 and the vapor pressure determination according to Grabner as set forth in DIN 51439.
The method of Reid proceeds from a vapor pressure curve wherein the fuel vapor pressure is plotted double-logarithmically (log—log) against the fuel temperature. In this plot, parallelly displaced linear curves result for different fuel qualities or fuel types. The method functions to determine, in advance, the rated pressure, which results when storing fuel in a vessel, and therewith determines the required pressure tightness of the vessel.
In the Reid method, first the maximum surface temperature of the fuel is determined which results during the storing operation. The corresponding vertically arranged temperature line in the above-mentioned Reid diagram intersects the Reid vapor pressure line of the particular fuel at a specific point. From the curve, the initial vapor pressure is determined via horizontal extrapolation starting from the above-mentioned intersect point. The value 14.7 is subtracted from this start value and this yields the above-mentioned required rated pressure of the vessel for storing fuel without vapor losses.
In contrast to the above, the method of Grabner is based on double-linearly plotted vapor pressure curves. Otherwise, however, one proceeds as in the Reid method so that the Grabner method is not explained further here.
The above-described known analysis apparatus and methods are little suited for the use in motor vehicles because of the required measuring complexity and evaluation complexity. These methods are especially not suitable for determining the fuel vapor pressure in vehicle operation and especially not for doing this exclusively with existing vehicle on-board means.